1. Field of the Invention
This invention relates to a four zone semiconductor device provided with guard zones to protect against reversal failure.
2. Description of the Prior Art
Such a semiconductor device is known for example from Swiss Patent No. 548,113. It constitutes a reverse-conducting thyristor, and consists essentially of a reverse cut-off thyristor with a diode in inverse parallel integrated in the same silicon wafer. As is implied by the name, such semiconductor devices have no cut-off capability in the reverse direction. In the forward direction, they function in the usual manner, that is to say they cut off the applied voltage until a firing pulse is applied via the control electrode. Such semiconductor devices are advantageously used for example in chopper circuits in electric drives (cf. for example IEEE Trans. Ind. Appl. IA-9 (1973), 236-247).
Semiconductor devices of the stated kind have the more particular advantage that the reverse current flows almost entirely via the integrated diode, and the device is accordingly capable of cutting off again more quickly upon transition to the forward cut-off condition.
However, a problem that arises with such semiconductor devices is that the charge-carriers present in the reverse conducting condition, which flow in the region of the integrated diode, exhibit a tendency to diffuse into the thyristor region upon transition to the forward cut-off condition (that is to say upon voltage reversal) and to cause the component to fire erroneously, with the result that in many cases the component is destroyed (known as reversal failure). Attempts have been made to counter this risk by providing a distance of at least two carrier-diffusion lengths between the emitting regions of the diode and of the thyristor for the purpose of forming a guard zone (Swiss Pat. No. 548,113), or by providing a channel in the protective zone between these regions (IEEE op. ct., FIG. 2). However, these measures have not proved to be sufficient.